Voltage reference tube



' June 12 1951 k CARNE 2,556,254

VOLTAGE REFERENCE TUBE Filed May 15, 1947 E jfzf; (We/251v CL A 5 $425Gaza-wr- 3nventor 622%: Che/ve- Gttorueg Patented June 12 1951 VOLTAGEREFERENCE TUBE Gerald G. Carne, Rockaway, N. J., assignor to RadioCorporation of America, a corporation of Delaware Application May 15,1947, Serial No. 748,270

6 Claims.

My invention relates to a gaseous discharge device and more particularlyto an improvement in a voltage reference tube.

Since the voltage drop across a cold cathodeglow discharge tube issubstantially independent of the current through the tube over a widerange of current values, tubes of this type are used in voltageregulator circuits in applications where it is necessary to maintain aconstant D.-C. output voltage across a load, independent of loadcurrents and moderate line-voltage variations. Such a regulated powersupply finds application with laboratory test equipment, as a substitutefor. batteries, or to provide a constant source of voltages for plate,screen or grid bias of vacuum tubes.

One of the most serious drawbacks to the use of known glow dischargetubes in D.-C. voltage regulator circuits for some applications, is theabrupt changes or jumps in the voltage of the tube in the order oftenths of volts. These jumps in voltage are observed to be a result ofthe oathode glow shifting from one section-of the cathode area toanother.

Another type of instability during the operation of certain kinds oftubes of the glow discharge type, takes the form of a slowly increasingvoltage druing the operational life of the tube; The voltage of suchtubes has been known to increase an amount of 5 volts in 200 hours oftube operation. This voltage change requires constant adjustmentwhenever measurements are to be made. Such a voltage change has proveddetrimental in some voltage regulator systems. This slow shift of tubevoltage is due to a slowly changing cathode work function.

It is therefor an object of my invention to provide a glow dischargetube of improved operation.

It is also an object of my invention to provide an improved glowdischarge tube in which the abrupt changes in tube voltage areeliminated.

It is also an object of my invention to provide a glow discharge tubehaving a constant voltage characteristic during the operational life ofthe tube.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims, but theinvenhaving an evacuated envelope l0.

tion itself will best be understood by reference current flow of lmilliampere.

a glow type discharge tube Within the envelope It there is enclosed agaseous medium comprising principally of a mixture of neon and argon.The envelope [0 has a tubular configuration in which the upper end isclosed by sealing off at It after exhaust and filling With gas. Thelower end of the tubular envelope is closed by having a glass buttonstem I2 sealed at its periphery to the. tubular envelope H3. Sealedthrough the bottom of the glass stem base l2 are a plurality of metalbase pins l8 which extend The tube of Fig. 1 is into the envelope andform electrical leads to the.

several electrodes enclosed therein. Supported from the stem base l2 ofthe tube is an annular cathode plate electrode it having an openapertured center portion [5. The cathode plate I4 is supported by metalrods It sealed through the glass stem 12 as extensions of respectivebase pins l8. Support rods it are fixed to the under side of the cathodeplate. Also, supported from the button stem I2 is an anode rod electrode20 extending coaxially within the tube envelope I 0 and through thecenter of the cathode opening [5. The anode rod 20 is spaced from thecathode plate M. The lower end of the anode rod 20 is supported belowthe cathode plate It by a transverse metal support 24 which in turn ismounted on the button stem l2 by support rods 26. Rods 26 may also beextensions of respective base pins l8 sealed through the glass stem 12.The upper end of the anode rod 20 is maintained in alignment coaxialwithin the tube envelope [0 by a spacer mica 28. A rivet-sleeve 30 iswelded to the top of the anode rod 20 and fastened by metal fasteners 3|to the mica spacer 28. The mica spacer 28 also supports a getter supportwire 32 formed as a horse shoe loop. Between the ends of the wire loop32 a channel member 33 is fixed. This channel member contains the gettermaterial, which is flashed during manufacture, as is well known in theart. Mounting the getter support loop 32 on the mica is a metal tab 34,mechanically looked through the mica.

The operation of the tube is such that, when appropriate voltage drop isestablished across the cathode electrode l4 and the anode rod 20, a glowdischarge will occur. By providing appropri-ate metal for the emittingcathode surface of electrode M and an appropriate mixture of rare gasesat a predetermined pressure within the tube envelope ill, it is possibleto provide the desired tube characteristics. For example, it was desiredto operate the tube of Fig. 1 with an operating voltage of volts acrossthe tube and a To provide a swam cathode emitting surface having asufficiently low work function as well as long life, misch metal is usedto coat the upper surface of the cathode electrode M. A gas mixture ofaround 99% neon and 1% argon is maintained at around 37 millimeterswithin the tube. These conditions of gas mixture and pressure as well ascathode surface will provide approximately a '75 volt drop across thetube and a normal cathode current density of 1 milliampere per squarecentimeter of cathode surface area. It is not necessary to be limited tomisch metal for determining the work function of the cathode [4 orto'the percentagesof gas mixture, or to the gas pressure -listedabove.Other materials such as barium or caesiated antimony, for example,having anxappropriate work function may be substituted for the mischmetal coating on the cathode surface 14. Also, other mixtures of othergases may be used at corresponding pressures to provide similaroperating characteristics of the tube of Fig. i. The effects of varyingthe conditions and materials in tubes of this type are well known tothose skilled in the art.

Fig. 2 discloses a well known relationship between the current andvoltage of a glow discharge tube of the type described for Fig. 1. Ifthe current is maintained suficiently low by means of an externalresistance, the discharge covers only a portion of the cathode surface il. An increase in the current through the tube causes the discharge tospread over a greater portion of the cathode surface so that the currentdischarge per unit area or the current density of the tubedischargeaswell as the voltage drop across the tube will remain constantduring this period of current increase. This condition is shown in Fig.2 by the curved portion between the points A and B. This portion of thecurve illustrates a condition of a constant normal current density. Thecurve between A and B is characteristic of glow discharge tubes and thisrange of operation is conventionally utilized in voltage regulatorcircuits where it is necessary to maintain a constant D. C. outputvoltage across a load. However, it is well known that one of the mostserious difficulties in the use of glow discharge tubes operated withinthese limits is the rapid shifting or the abrupt small changes involtage across" the tube during tube operation. These jumps are notaccumulative but merely are changes back and forth in the order oftenths of volts from the average tube voltage. Under some conditions,these abrupt changes of the tube voltage can be ignored. However, underother conditions, such rapid shifts in the voltage of the tube cannot betolerated. It has been necessary then to design a glow discharge tube inwhich these rapid shifts in tube voltage are eliminated.

The principal cause for the rapid changes in tube voltages describedabove is the jumping of the glow discharge from one portion of theoathodesurface to another unused portion of the surface, having adifferent work function. It is well known in tubes of this type thatwith the current density remaining constant, the total tube currentmaybe increased until the glow discharge covers the entire surface ofthe cathode electrode. At this point the tube has reached the criticalcondition at which it is no longer possible to increase the current flowthrough the tube without a corresponding increase in the tube voltageand in the current density of the cathode discharge. This critical pointis represented by B in Fig. 2 and the curve from B to C indicates thecharac- 4 teristic rise in tube voltage as the current flow through thetube is increased.

I have designed the tube in Fig. 1 to be operated on that portion of thecurve between points B and C. The current density discharge from thecathode surface changes beyond the critical point B and in relation to achange in the tube voltage. This portion of the curve between B and C isknown as that of abnormal current "density. I have found, that when aglow discharge tube is operated at a point in the abnormal portion BC ofthe curve, the rapid changes in tube voltage as described above areeliminated. When operating in this abnormal range the glow dischargeorcathode emission takes place from the entire cathode surface l4 andthere is no unused portion of the cathode surface to which the cathodeglow or discharge may shift. To maintain the cathode emission surface ata constant'area, I have coated the under surfaceof the electrode platelfwith' an aluminum coating. This aluminum coatingprevents the dischargeglow from spreading to the under surface and causing variations in'thetube voltage.

In operating a tube similar to that of 'Fig. 1, it may be connected inseries with a resistance between theterminals of 'a source of regulatedD. C. voltage. The resistance may be adjusted to a valuesuch that the"current flow through the gas tube may be large enough to provide acathode glow discharge over all of the upper surface area of the cathodeelectrode l4 covered by the misch metal and such'that the cathodedischarge has an abnormal current density. Under these conditions, thetube will be operating in the'current density range represented'by curveBC of Fig. 2.

As mentioned above, the work function of misch metal is such that thevoltage drop across the tube is around 75 volts. The normal currentdensity of a misch metal coated electrode at this voltage will beapproximately'l milliampere per cm. Since it is desired that thetotal-current flow through the tube of Fig. 1 shall be' approximately 1milliampere, the cathodesurface I4 is designed to have an area of 0.8cm. This cathode surface area will result in the desired tube current of1 milliampere'when the tube is operlent results.

than 0.05 volt.

ated within the abnormal current density range.

Tubes, which have been'cons'tructed' according to the above describedmannenhave given excel- These tubes after being properly processed andaged have 'beenoperated for test periods'greater than 100-hours.-.By'operating these tubes entirely within the "abnormal" currentdensity-range during'these test periods, I have observed no abruptchanges in voltage greater In operation, tubes of" this type have provedof great value in voltage-regulator circuits and for-"voltage' referenceapplications. In such applications a constant current flow which canvary only to the" order'ofmicroamperes is "maintained through the tubeby acontrolcircuit. When the tube is being operated-"within "theabnormalcurr'ent densityrange', such changes voltage drop during thelife ofthe tube. I have notedtubes of the above described type to have avoltage shift up to 2 volts and greater over a period of 200 hours oftube operation. This slowly shifting tube voltage is undesirable as ithasto. be continually adjusted. for.

Furthermore, under certain conditions the tube cannot be used due tothis voltage change. It appears that the cause of such a change in thetube voltage is due to a changing cathode work function.

operation.

tively charged it apparently draws to it positive gas.;ions producedduring the tube operation. This results in a bombardment of the innersurface of the envelope walls by the positive ions and it-appears thatthis bombardment actively releases occluded gases from the envelopewalls.

These occluded gases consist in a large part of oxygen which whenreleased within the tube poisons the emitting surface of the cathodesufii ciently. to change its work function. This then .is effective incausing the gradual increase in the voltage drop of the tube.

I have found, however, that if the walls of the tubular envelope ID arecoated with a metallic film 36.that this gradual shift in tube voltageis eliminated. .The best metallic film is that of the same metal as thatof the cathode emitting sur-- face. In Fig. 1 the metallic film is acoating .Ol f misch metal identical with that covering the cathodesurface 14. If other metals are used for the cathode emission surface,the same meta: used for the metallic film 36 produces best re sults.

It isnot entirely clear just how this metallic film prevents the changein tube voltage. How ever, a logical explanation may be that the metal"lic coating 36 on the inner surface of the glass walls prevents theliberation of gas from the glass surface by ion bombardment. Either themetal film 36 shields the glass wall effectively from the ionbombardment or the metal film may form a means for sealing the poroussurface of the glass to prevent escaping of the gas during theconditions of tube operation. Also, another explanation of the functionof the metal coating on the inner surface of the glass tube is that themisch metal used may react chemically with the occluded gases releasedby the ion bombardment. Since the emitting surface of the cathodeelectrode is also covered by misch metal, any deleterious gas within theglass surface of the tube which may effectively poison the cathodeemitting surface by reacting therewith, must first come in contact withthe misch metal film on the inner surface of the tube. Thus, the gaswill react chemically with the misch metal coating instead of with themisch metal cathode emitting surface.

This metallic coating 36 on the inner surface of the glass dischargeenvelope Ill may be applied in any desirable manner. However, I havefound it advantageous to deposit the metallic film 36 by evaporation. Alump of misch metal is fastened to the anode rod 20 in a position shownby dotted lines .38. During the aging of the tube. the anode rod isconnected to the negative' terminal of a source of voltage while thecathode plate I4 is connected to the positive terminal. With theestablishment of an appropriate voltage between the negative rod 20 andthe positive plate M the misch metal lump 38 is bombarded with positiveions causing it to be sputtered and to evaporate.- This evaporationcontinues until all of the tubular wall surface below the mica disc 28is covered with a metal film. Furthermore, the cathode plate IQ iscovered with a uniform layer of the misch metal.

- The misch metal layer 36 on the tube surface is establishedsufficiently below the level of plate. [4 so that the region of the glowdischarge between .the. cathode M and anode 20 is completely enclosed bythe metal coating 38. It has. been found that the metal coating shouldcompletely cover all portions of the glass tube l 0 whichmay besubjected to the positive ion bombardment. This extent of the metalcoating from the mica 28 to a point below the cathode disc I4 has provedsufficient to eliminate the freeing of the occluded gases by positiveion bombardment.

Theglow discharge tube described above which is designed according to myinvention has eliminated two sources of trouble during tube operation,vby solving the problem of rapid voltage shifts and also the problem ofgradual tube voltage change. I have a gas discharge tube which hasgreater application than the conventional glow discharge tube. Thisimproved tube may be depended upon to maintain a more constant voltagein applications where shifts of tube voltage in the. order of tenths ofvolts have proved detrimental. 'Also, this tube which maintains a moreconstant voltage drop has found much valuable use as a voltage referencesource in equip- .ment where voltage changes found in conventional tubesprove detrimental.

.While certain specific embodiments have been illustrated and described,it will be understood that various changes and modifications may be madetherein without departing from the spirit and scope of the invention.

What I claim as new is:

1. A glow discharge device comprising an envelope, a gaseous mediumwithin said envelope, an anode electrode and a cold cathode electrodeenclosed within said envelope to provide a glow discharge therebetween,a substantial portion of the interior surface of said envelope beingexposed to said glow discharge, a metal coating on said cathodeelectrode for determining its work function, said coating havinginherent gettering properties, and a coating of the same metal onsubstantially the entire inside surface of said envelope exposed to saiddischarge to inhibit the liberation of gas from said surface.

2. A glow discharge device comprising an envelope, a gaseous mediumwithin said envelope, an anode electrode and a cold cathode electrodeenclosed within said envelope to provide a glow discharge therebetween,a substantial portion of the interior of said envelope being exposed tosaid discharge, a misch metal film deposited on a portion of the surfaceof said cathode electrode for determining the work function of saidcathode electrode, .a misch metal coating on substantially the entireinside surface of said envelope exposed to said discharge to inhibit theliberation of gas from said surface.

3. A glow discharge device comprising an envelope, a gaseous mediumwithin said envelope, a. rod-shaped anode electrode mounted within said7 envelope,ia fiateold cathode electrode within said envelope and'spaced from said anode electrode to provide a glow discharge.therebetween, a metal deposited on aportion of .the surface of saidcathode electrode for determining thework'function,

of said cathode electrode, said metal having. inherent getteringproperties, and a metallic coating on the entire inside surface ofsaidenvelope exposed to saiddischarge to inhibit the liberation of gas fromsaidsurface, said metallic coating ineluding the. same ;metal. deposited.,on..said :cath- .ode portion.

4.. A glow discharge device comprising an envelopehavinga tubular.portion, a gaseous medium within said .envelope, ;an annular ldisc'shaped.

cold cathode electrode coaxially mounted within said tubularenvelope-portion, a .misch metal coating on a portion of the surface ofsaid. cathode electrode for determining. its .work function,

an anode electrode. rod 'withinusaid tubular en velope portion,.saidanode: rod co'axially extending through the open center of'said annularcathode electrode and spaced therefrom to provide a glow dischargetherebetween, and a misch metal coating on substantially the. entireinside surface of said envelope portion.

5. A glow discharge device'comprising an envelope having a tubularportion, 'a gaseous medium within said envelope, a.circular platecathode electrode coaxiallvmounted within said tubular envelope.portion,.said circular cathode plate having an aperture at....the-.center thereof, an anode electrode. rod withinisaid tubular envelopeportion, said anode rod coaxially extending through the aperture. ofsaid cathode plate and spaced therefrom, aimisch metal coating on oneside of said cathode plate to determine .the: work functionof saidcathode: electrode, an aluminum coating on the other side: of saidcathode plate to inhibit a discharge betweensaid other side of 4 thecathode plate and saidzanode electrode, and

. a f'mischcmetal; coatinglon theiri'nside-lsurfaoe of said: tubular 1envelopeportion: in" theiiregion-i of said discharge.

cathode. plate andrsaid.anoderelectrodepland a coating of achemicallyr'active.'metal on'the inside .surface. of saidLtubularenvelope portion in theregion of said discharge.

GERALD G. CA-RNE.

REFERENCES. CITED The following 'referencesare ofrecord inthe file ofthis patent:

.UNITED. STATES PATENTS Number Name Date 1,128,817 ".Pickard ;?Feb. 16,-1915 1,571,257 Freeman Feb. 2, 1926 1,689,338 Harris 0012.30, 19281,760,524 Rentschler .-May 27,1930 2,056,662 dFoulke -Oct'. 6,19362,103,031 Foulke .Dec. 21, 1937 2,331,398 Ingram Oct; 12, 19431*"01'1515'IC1N PATENTS Number Country 7:1 Date 272,343 GreatBritain-;June 17,1926 627,784 3 France -,.June14, 1927

